Question

Mendelian Disorders
Read the following passage carefully and answer the given questions.
Genetic disorders may be grouped into two categories - Mendelian disorders and Chromosomal disorders. Mendelian disorders are mainly determined by alteration or mutation in a single gene. These disorders are transmitted to the offspring on the same lines as in the principle of inheritance. The pattern of inheritance of such Mendelian disorders can be traced in a family by pedigree analysis. Most common and prevalent Mendelian disorders are Haemophilia, Cystic fibrosis, Sickle-cell anaemia, Colour blindness, Phenylketonuria, Thalassemia, etc. The Mendelian disorders may be dominant or recessive. By pedigree analysis, one can easily understand whether the trait in question is dominant or recessive. Similarly, the trait may also be linked to the sex chromosome or an autosome.

Which one of the followings is an example of an inborn error of metabolism and inherited as autosomal recessive trait?

• A. Phenylketonuria
• B. Thalassemia
• C. Haemophilia
• D. Colour Blindness

Hint:

Think of a missing enzyme in a metabolic pathway on an autosome with recessive inheritance.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
An inborn error of metabolism is a genetic disease where one enzyme of a metabolic pathway is missing or faulty. The question also needs the inheritance to be autosomal recessive.

Step 2: Check option 1, Phenylketonuria:
In this disease the enzyme phenylalanine hydroxylase is missing, so phenylalanine cannot change to tyrosine. It builds up and harms the brain. The gene is on an autosome and both copies must be faulty, so it is autosomal recessive. Option 1 is right.

Step 3: Check option 2, Thalassemia:
It is a defect in the amount of globin chains made. It is a blood disorder and not an enzyme (metabolic) defect, so option 2 is wrong.

Step 4: Check options 3 and 4:
Haemophilia and colour blindness are X-linked recessive traits. They are not autosomal, so both are wrong.

Final Answer:
Phenylketonuria is the inborn error of metabolism with autosomal recessive inheritance, which is option 1.